kbuild: clean up objtool_args slightly

The code:

  $(if $(or $(CONFIG_GCOV_KERNEL),$(CONFIG_LTO_CLANG)), ...)

... can be simpled to:

  $(if $(CONFIG_GCOV_KERNEL)$(CONFIG_LTO_CLANG), ...)

Also, remove meaningless commas at the end of $(if ...).

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>